To test the
possibility of differential selection of colour morphs on different backgrounds,
colour proportions of juvenile and adult shells were compared on trees with
abundant foliage and on those with sparse foliage at the levels occupied by the
snails. Selection by the morphs of the background against which they are most
cryptic may promote polymorphism (Endler, 1978), so the distribution of shell
colours on bark and leaf substrates within trees was investigated.
By releasing populations of known colour frequencies on isolated trees and collecting the surviving snails after a period of time, it was possible to test for frequency dependence in the disappearance of colours.
If apostatic selection operates on morphs which are cryptic on different backgrounds, their frequencies at equilibrium will be related to the proportional areas of these backgrounds which are available in the environment.
To test this hypothesis, the
manipulative experiment was carried out on trees with abundant foliage, and on
others which were partially denuded of leaves.